1. Field of the Invention
The present invention relates to ultrasound therapy devices with automatic control power radiated to the patient under changing coupling conditions, or to other applications of ultrasonic wave generators where precise control of radiated power under varying load conditions is required.
2. Description of Related Art
Therapeutic ultrasound units currently on the market employ high frequency oscillators and power amplifiers to generate a high frequency electrical signal that is then delivered to a piezoelectric transducer housed in a handheld applicator. The transducer converts the electrical signal to ultrasonic energy at the same frequency. The ultrasonic energy is then transmitted to the patient by applying a radiating plate on the transducer against the patient's skin.
Out of the total power of the electrical signal delivered to the transducer, only a part is actually radiated to the patient's tissue as ultrasonic energy. The other part of the total power is dissipated in the transducer and parts of the applicator in the form of heat. As the applicator is moved over a treatment site, the acoustic coupling to the patient's body changes, resulting in a change in the proportion of the power radiated to the patient relative to the power dissipated in the transducer. This coupling efficiency change is caused by changes in acoustic impedance as different types of tissue are encountered, and as air, whose acoustic impedance is much different than that of tissue, enters the space between the skin and the applicator.
The typical therapeutic ultrasound unit of the prior art allows for measurement and manual or automatic control of the total electrical power delivered to the transducer. However, as mentioned above, due to changing coupling efficiencies as the applicator is moved, the amount of power delivered to the transducer is often an inaccurate indication of the actual amount of power radiated to the patient. These prior art systems which control the amount of power delivered to the transducer have power meters or power control systems calibrated corresponding to radiated power for the average good coupling conditions. These conditions are typically simulated by radiating ultrasonic energy into de-gassed water, or under other simulation conditions. These calibration techniques, based on average good coupling conditions, are highly inaccurate in many practical uses of therapeutic ultrasound equipment. The proportion of the power radiated to the patient of the total power delivered to the transducer changes significantly under real treatment conditions, resulting in a significant error in these prior art techniques for determining the amount of radiated power to a patient.
Furthermore, these prior art systems are equipped with timers that can be programmed for fixed treatment time. This fixed treatment time is selected in response to a desired dosage of ultrasonic energy for given therapeutic needs. However, as the power radiated to the patient changes during the treatment in an uncontrolled way due to changes in coupling efficiency, the actual radiation dose received by the patient over the treatment time cannot be accurately assessed.
Therefore, the prior art systems have been unable to measure the power radiated to a treatment site instantaneously, or to effectively determine the total radiation dose given during a treatment cycle.
The therapeutic ultrasound units of the prior art typically do not provide an indication of coupling of quality. Some units provide an indicator of the decoupled condition, or a four level coupling indicator. Very few units provide wide range, high resolution coupling meter. Those that do are still limited to the type of applicators with which they have been factory calibrated to operate.
These coupling indicators or meters actually indicate changes to the radiation power as the coupling changes. The units of the prior art are not capable of maintaining constant radiating power while monitoring changing coupling conditions.
Also, in prior art systems, transducer overheating in uncoupled conditions is addressed. When the coupling efficiency of a transducer approaches zero, such as when the applicator has been tilted, or moved to an area With insufficient amount of coupling gel, essentially all of the power delivered to the transducer is dissipated in heat, warming up the applicator. This can result in overheating and permanent damage to the transducer This problem is particularly severe in the prior art units that employ a power control loop maintaining constant power to the transducer such as described in U.S. Pat. No. 4,368,410, to Hanoe, et al.
To prevent overheating, some prior art units employ a warning signal that comes on when an uncoupled condition is detected and the operator is required to shut the power down. Other units employ temperature sensors mounted inside the applicator to detect overheating and automatically shut the power down. The approach involving a warning signal in the uncoupled condition does not protect the applicator against human error. The technique involving shutting down the power in response to overheating, requires a long cooling period before the unit can be put in service again.
Prior art systems also require frequent calibration. Even under ideal controlled coupling conditions, a nominal radiation power accuracy cannot be guaranteed unless the unit undergoes periodic calibration. This is true because the parameters of the ultrasonic transducers that influence the power ratio change with time. Also, any change in the type of applicator, or the applicator within the same type, necessitates further power calibration.
In ultrasonic generating units, the frequency of the oscillator has to be tuned to the resonant frequency of the transducer. Most of the units on the market employ manually tuned oscillator that is factory adjusted for operation with a specific applicator. Any change of applicator, such as replacement of a damaged applicator, requires re-tuning and power calibration that can only be done in a specialized laboratory. Since the resonant frequency of the transducer changes as it ages, a periodic re-tuning of the unit is also required.
Some units employ phase lock loops that continuously update oscillator frequency to achieve zero phase error between voltage and current driving the transducer, such as described in U.S. Pat. No. 4,302,728, to Nakamura. Using the phase lock loop eliminates the need for periodic re-tuning. It becomes impractical, however, when self tuning with a wide range of different types of applicators is required. For instance, standard applicators currently in use, operate with either 1 MHz or 3 MHz as the center of ultrasonic drive frequency ranges. Each of these frequency ranges requires a different type of phase shift circuit for the phase look loop. Thus, a single control unit cannot be used for either type of applicator.
Another problem in the design of ultrasound equipment arises because the applicator radiating surface causes an unpleasant feeling when applied against a patient's skin, unless it is warmed up. It is desirable to keep the applicator at a temperature elevated to approximately the temperature of the human body. Some elements of the prior art offer applicator warming feature implemented by means of a resistive heating element mounted inside the applicator and continuously powered. This approach has the disadvantage of being expensive to manufacture and in absence of power control offering long warmup time and low temperature stability.
Accordingly, it is desirable to provide a system for controlling power delivered to an ultrasonic applicator that provides greater control over actual dosage of ultrasonic energy, can handle a wide variety of applicator types without expensive, factory re-calibration or tuning, and overcomes other problems discussed above of prior art ultrasonic therapy units.